This invention relates to an oil-sealed vacuum pump which includes an oil circuit for supplying bearings and the pump chamber with oil as well as a valve arranged in the oil circuit for shutting off the oil supply to the pump chamber when the pump is at a standstill.
A vacuum pump of the above-outlined type is disclosed in British Pat. No. 875,444. The oil pump of the oil circuit of the vacuum pump disclosed in the British patent draws oil from a sump situated in the pump housing and drives the oil to a check valve-type arrangement whose closure member is biased by a very weak spring so that the oil pressure in the oil circuit is only slightly above atmospheric pressure. From the valve which opens when the pressure of the closing spring is overcome, the oil delivered in excess by the oil pump passes through the bearings of the pump shaft into the pump chamber and therefrom is reintroduced into the sump by a discharge valve.
It is a disadvantage of the above-outlined vacuum pump structure that the oil cannot be admitted to the bearings with a pressure which is significantly above the atmospheric pressure although such a higher oil pressure would be desirable for a reliable and continuous lubrication of the bearings. It would be thinkable to select a stronger spring for the check valve to cause an increase of the oil pressure in the oil circuit. This, however, would mean that the oil serving as a seal for the rotary piston is injected into the pump chamber with a continuous high pressure. The disadvantage of such an arrangement resides in the fact that at high suction pressures unnecessarily large oil quantities would be injected into the gases delivered in large quantities. This not only unnecessarily increases the oil consumption during the operation of a pump working at high suction pressures but also would mean an increased environmental pollution because of the high oil content in the gases expelled by the pump.